Associative thickeners are hydrophobically-modified water soluble polymers that offer improved rheological performance over traditional thickeners. They represent a relatively new technology in the field of water-soluble polymers and have received considerable attention, particularly over the last decade. The molecular structure requirements necessary to induce association have been discussed in the literature with respect to a variety of synthetic polymers.
Thickeners, in general are natural or synthetic polymers used to modify the viscosity characteristics of fluids. While they are most often used as components of water-based systems, such as in latex paints, they can sometimes be used in nonaqueous systems. The most prominent category of thickening agents is cellulosic polymers, such as carboxymethyl cellulose (CMC) and hydroxyethyl cellulose (HEC).
In the early 1980s, a new type of synthetic thickener, called an associative thickener, was developed. The object was to improve some of the performance deficiencies of cellulosic thickeners in latex paints. Associative thickeners are water-soluble polymers that have been modified by the addition of controlled amounts of certain hydrophobic groups. In solution, these hydrophobic groups are believed to associate with each other and also, in some cases, with pigment and latex components that are present in paint and other coating-type formulations. Such inter-molecular associations are believed to account for a thickening action which cannot be explained by the molecular weight of the thickening agent.
U.S. Pat. No. 4,228,277, Landoll (1980) discloses modifying cellulose ethers, such as hydroxyethyl cellulose, with long chain alkyl groups having 10 to 24 carbon atoms, in order to improve the viscosifying effect of the cellulose ethers. Modification is by reaction with long chain epoxides or halides. See also Gelman, et al., "Viscosity Studies of Hydrophobically Modified (Hydroxyethyl) Cellulose", ACS Adv. in Chem. Series, pages 101-110 (1986) for a discussion of associative thickening with hydrophobically modified hydroxyethyl cellulose.
Synthetic compositions based upon vinylpolymers had been prepared much earlier and these compositions may have operated in the same manner as associative thickeners.
U.S. Pat. No. 3,779,970, Evani, et al., (1973) discloses thickening latex coatings such as paints with an ester of a carboxylic polymer in which some of the pendent carboxylic acid groups have been esterified with a nonionic surfactant, such as nonyl phenol, condensed with excess ethylene oxide to form a hydrophobic group spaced from the polymer backbone with a hydrophilic polyethylene oxide. The paint is said to have improved flow and leveling properties. Vinyl acetate can be copolymerized to prepare the carboxylic polymer.
More recently, attention in this field appears to have been directed toward modified urethane polymers. U.S. Pat. No. 4,079,028, Emmons, et al., (1978) discloses thickening aqueous systems, such as water-based paints, with a low molecular weight polyurethane having at least three hydrophobic groups connected by hydrophilic polyether groups. The molecular weights of these polymers are said to range from 10,000 to 200,000, preferably 12,000 to 150,000. Polymers are made in non-aqueous media involving polyether polyels and isocyanates. It is said that these products are potentially useful in the manufacture of paints, textile coatings, printing inks, herbicides, topical medicines, cosmetics and hair conditioners.
U.S. Pat. No. 4,426,485, Hoy, et al., (1984) describes what is represented as an improvement over the '028 polymer thickeners in the development of polymers having hydrophobic segments "bunched" together rather than distributed along the polymer chain. Polymers exemplified include condensation polymers of polyethylene glycol and toluenediisocyanate which are reacted with a hydrophobic diol.
Another approach which has been made to obtain improvements over the '028 thickeners has been to develop blends of polymers. U.S. Pat. No. 4,507,426, Blake (1985) describes thickening agents for paints made by blending urethane polymers as described by the '028 patent with an alkali-soluble aqueous emulsion polymer made from a carboxylic acid monomer, such as acrylic acid, a nonionic vinyl monomer, such as alkyl esters of acrylic acid, and a nonionic vinyl surfactant ester, such as alkylphenoxypoly(ethyleneoxy) ethylacrylates.
U.S. Pat. No. 4,735,981, Rich, et al., (1988) describes still another improvement on the blended thickeners of the '426 patent in which a nonionic urethane monomer is used instead of the nonionic vinyl surfactant ester.
A new class of polymers known as hydrophobically modified amine functional polymers is described by U.S. Pat. No. 5,086,111, Pinschmidt, et al., (1992). This patent discloses monoaldehyde modified poly(vinylamine) which can be a homopolymer or a copolymer with polyvinyl alcohol. The use of aldehyde modifiers containing as high as 21 carbon atoms is disclosed, but the discussion centers around monoaldehydes containing from 1 to 7 carbon atoms. The products are said to be useful as flocculants, in emulsifiers and as protective colloids, and also as epoxy resin and polyurethane crosslinking agents. The molecular weights of these polymers range from 10,000 to 7.times.10.sup.6 and flocculation of bentonite aqueous suspension is demonstrated.
Copending patent application Ser. No. 07/803,256 filed Dec. 5, 1991 and now U.S. Pat. No. 5,185,083 describes using the same polymers of the '111 patent in wastewater treating in order to assist in separating the solids from aqueous suspensions. Other utilities disclosed include crosslinking polymers, use in emulsifiers and as protective colloids. Aldehyde modifiers containing 2 to 8 carbon atoms are specifically named. In copending application Ser. No. 07/826,330 filed Jan. 24, 1992 and now U.S. Pat. No. 5,232,553 polymers as described by the '111 patent, particularly modified with aldehydes having 2 to 12 carbon atoms, are described as useful in papermaking to retain fines, especially in recycle waste. A waste paper pulp slurry is described treated with a C.sub.12 -modified polyvinyl amine hydrochloride having a molecular weight of 6.4.times.10.sup.5 and it is shown that the polyvinyl amine modified with a C.sub.4 aldehyde provides better performance in this particular service than the C.sub.12 -modified polymer.